The present invention relates to a method for the preparation of an organopolysiloxane or, more particularly, to a method for the preparation of an organopolysiloxane, such as a silicone fluid, free from any trace amount of the acid catalyst residue and still containing a greatly reduced amount of the silanolic hydroxy groups in a process of the siloxane rearrangement reaction of organosiloxane oligomers to effect polymerization and equilibration by using an acidic catalyst such as sulfuric acid and fuming sulfuric acid.
It is a well established process in the art of organopolysiloxanes or so-called silicone products, such as silicone fluids and the like, that the organopolysiloxane is prepared by subjecting organosiloxane oligomers to the siloxane rearrangement reaction to effect polymerization and equilibration by using sulfuric acid or fuming sulfuric acid as the catalyst. It is also known in this process that fragments of the acid catalyst are contained in the product organopolysiloxane as combined to the molecular chain ends of the organopolysiloxane. An organopolysiloxane product containing such an acid catalyst residue is disadvantageous in respect of the unpleasant odor and corrosiveness when it is used as an ingredient in a cosmetic or toiletry preparation or as a material in various electronic and electric devices and instruments. Therefore, the acid catalyst residue contained in an organopolysiloxane should desirably be removed therefrom as completely as possible.
Accordingly, it is conventionally undertaken to wash the organopolysiloxane as polymerized thoroughly with a large volume of water so that the acid catalyst residue can be removed to a considerable extent because the acid residue combined to the molecules of the organopolysiloxane is susceptible to hydrolysis. This conventional method, however, never provides a complete solution of the problem since, even by setting aside the problems due to the troublesomeness of the procedure and disposal of the so large volume of acidified washing, removal of the acid catalyst residue cannot be complete enough always leaving a small but non-negligible amount of the acid residue as contained in the organopolysiloxane. Moreover, thorough washing of the organopolysiloxane with a large volume of water to remove the acid catalyst residue unavoidably introduces silanolic hydroxy groups into the organopolysiloxane in a correspondingly large amount. When such a silanol-containing organopolysiloxane is employed, the silanol groups obtained from the dehydration condensation during use to produce water as the condensation product is acidified by the trace amount of the acid residue therein and causes serious problems of corrosion of metallic surfaces or breakdown of electric insulation. Thus, it is eagerly desired to develop a convenient and reliable method for the removal of the acid catalyst residue from an organopolysiloxane prepared by the acid-catalyzed reaction to an as high as possible extent of completeness without increasing the content of the silanol groups or rather by decreasing the content thereof.